This invention pertains to an apparatus and method for controlled chemical blending in general. In particular, the invention provides for the high speed chemical blending of a variety of chemicals through a pair of automatic inlet valves, one with a large inlet orifice and one with a relatively much smaller inlet orifice. The processes used in semiconductor manufacturing require the blending of two or more chemicals in the ordinary case. The correct percentage of each chemical in the blend is critical and typically must be within a tolerance of 0.5% by weight. Not only is the correct percentage of the blend critical, but rapidly obtaining and maintaining the mixture is important as well. A third problem faced is the need to change the mixture at any time. The prior art discloses a variety of devices designed for mixing. The Falcoff, et al patent, U.S. Pat. No. 4,403,866 discloses a process for making paints which meet the requirements of a standard paint formula loaded in a computer. A colorimeter is used to determine whether the paint is within the tolerance values of the standard paint.
The Borrow patent, U.S. Pat. No. 4,509,863 discloses an apparatus for mixing milk with flavoring and fluoride. The device consists of two sets of three vessels. The three vessels are constructed so that their internal volumes are in the same ratio as the volumes of liquid in the final beverage.
The Beerier patent, U.S. Pat. No. 4,671,892 discloses a device for making soap on site by first weighing the materials to be mixed, then mixing them and transferring the mixture for storage at the end of a predetermined reaction time.
More directly related prior art patents are disclosed in Mackay, et al, U.S. Pat. No. 5,288,145, for a device that monitors the weight of a mixing tank as various switches are actuated to add the required ingredients. No precise requirements of mixing are disclosed and the control over dilution is accomplished by varying the speed of the pump adding the dilutant.
The Patel, et al patent, U.S. Pat. No. 5,340,210 discloses a device for blending chemicals which includes a single blending vat and a pump that includes forward and reverse directions and has a high and low speed function. The Patel vat is initially filled at high speed and, prior to the introduction of the full amount of chemical to be blended, is shifted to the low speed for the precise addition of chemical. This system is mirrored by the Major patent, U.S. Pat. No. 5,558,435 and the O'Dougherty, et al patent, U.S. Pat. No. 5,522,660.
Other means and methods exist for the creation of a proper chemical blend by dilution and otherwise such as disclosed in the Owczarz patent, U.S. Pat. No. 5,409,310, which utilizes aspirator injectors which have a suction port which draws from a mixing tank so that as a primary fluid flows through the aspirator injectors, the already diluted additive is further diluted.
A primary drawback of the chemical blending systems known in the art is the inability to create accurate chemical blends within a very narrow range of tolerance at high speed. Other drawbacks are the inability to obtain the desired chemical balance quickly and accurately and maintain a constant flow of the blend as required. Further, the prior art points out the need for a chemical blending system that also has the ability to rapidly and actively change chemical blends when desired. It, therefore, is an objective of the chemical blending system of the present invention to provide for a continuously high speed apparatus and method of creating chemical blends within a very small range of tolerance by weight that is capable of manipulation of the chemical blend so as to obtain and maintain the proper mixture over time and which can change the blend as desired.